MXPA99012015A - Divid diode high voltage transformer - Google Patents

Abstract

The present invention relates to a high-voltage split-diode transformer having a core, a primary winding and a high-voltage winding, in which it is disposed in chambers (CI-C12) of a coil winding former, in which chambers (Cl-C12) with the high-voltage winding are located below the primary winding, a conductive coating (15) is disposed on the surface of the internal cavity of the coil winding former, and by virtue of a corresponding arrangement and wiring of the cameras (Cl-C12), the oscillations that arise during the operation in the high voltage transformer induce capacitive currents on the conductive coating (15), the sum of the capacitive currents is approximately zero. This can be achieved, for example by virtue of a symmetrical arrangement and cabling of the chambers (Cl-C12) with respect to the diodes (3,4), the oscillations induce capacitive currents on the conductive coating (15) that occur in pairs with the same amplitude, but in antiphase, and through this they cancel each other. In particular, by virtue of an identical thickness of the lower part and approximately identical return numbers for all the cameras (C1-C12), the capacitive currents occur with a quantized amplitude, with the result that the values of the same can be defined in a simple way, since the parasitic capacitances (SC) are identical for all the cameras (Cl-C12). This arrangement allows the ground connection (G) to be omitted, although the shielding effect of the conductive coating (15) is retained.

Description

DIVIDED DIODE HIGH VOLTAGE TRANSFORMER DESCRIPTION OF THE INVENTION The present invention is based on a high-voltage split-diode transformer having a core, a primary winding and a high-voltage winding, which is arranged in chambers of a former for winding coils. The structure of a high-voltage transformer of this type and also the manner in which these chambers are coiled are explained for example in EP-BO 529 418. The high-voltage transformer of a television or computer monitor is a relatively expensive component. , so it is desirable to simplify its production, but without reducing its operational reliability. Patent application PCT / EP 98/03882, published after the priority date, has already specified a high-voltage transformer in which the high-voltage winding is located below the primary winding, between the primary winding and the core , with which this becomes considerably more compact, lighter and more economical. To avoid high-voltage spark discharges and corona effects, this transformer has an insulation, for example a conductive coating, between the coil winding former and the core. It is also desirable for the high voltage transformer not to emit as much interference radiation as possible, since, due to the high level integration of the semiconductor circuits, the frame of a television has become very compact in the meantime, and the irradiation of the tuner circuit is thus possible. In this case, it is particularly the high-voltage split-diode transformers which are problematic, since their high-voltage winding is on the outside, and has no shielding in any way, or the shielding is very complicated and troublesome. The measures for reducing this interference radiation or undesirable oscillations are described for example in EP-A-0 735 552 and EP-A-0 729 160. The object of the present invention, therefore, is to specify a high-voltage transformer of split diode of the type mentioned in the introduction, which is very compact and at the same time has good shielding of the interference radiation. This object is achieved by means of the invention specified in claim 1. Advantageous developments of the invention are specified in the subclaims. The high-voltage split-diode transformer according to the invention contains a core, a primary winding and a high-voltage winding, the high-voltage winding is located below the primary winding, or within the primary winding relative to the housing. In this case, the high-voltage winding is arranged in chambers of a winding-up formers, whose surface of the internal cavity between the winding-up formers and the core is provided with a conductive coating, thereby avoiding corona effects. Corona effects occur in particular if a high electric field is present in the air or in air inclusions, whereby ozone is produced, which is chemically highly aggressive and destroys the coil winding and / or insulation . The conductive coating makes it possible to completely screen the electric field between the high-voltage winding and the core, with the result that there are no air inclusions or air spaces with high electric fields between the conductive coating and the high-voltage winding during the operation of the high voltage transformer.

The conductive coating is advantageously a thin layer containing colloidal graphite. The layer can be applied in a simple manner by spraying a liquid spray agent, comprising colloidal graphite and adhesive in a solvent, onto the inner wall of the former to wind coils by means of a nozzle. The conductive coating can alternatively be a metallized film, which is hermetically supported on the inner wall of the coil winding former, or can be formed by encapsulating the interspace between the core and the coil winding forme with a conductive material. Additional details regarding the conductive coating are specified in PCT / EP 98/03882, to which reference is made herein. The diodes of the high-voltage transformer are located, in particular, not between or above the chambers with the high-voltage winding, but outside the chambers, with the result that the primary winding, taking into account a corresponding insulating layer , it can be arranged directly above the chambers, and it is tightly wound in such a way that the high-voltage winding is completely covered by the primary winding. As a result of this, along with the conductive coating on the underside of the coil winding former, a noticeable shielding for the high voltage winding is produced. It is appropriate, on the other hand, at least in the case of high voltage transformers that have two and four diodes, to connect one outer camera to ground, and to provide the other outer camera as a high voltage connection, with the result that the high-voltage transformer is also completely shielded laterally, and the upper part and the lower part in the case of a vertical design. For the shielding effect of the conductive coating, the latter must be connected to earth or connected to a constant electric potential. It has been shown, however, that the thin electrical cladding can not be connected by contact to a metal conductor without problems, since the conductor can only be fixed and not welded, and the conductor only allows contact at points, or only a very small surface of the conductive coating is connected by contact. Since the conductive coating has, in particular, a high impedance to avoid eddy currents, the contact point to the ground connection can be destroyed by compensation currents. A measure of the resistance through the conductive coating on the length of the coil winding formers provides resistance of between 20 kohms and 2 Mohms, for example, depending on the design, This ground connection can be avoided, however, if the cameras are they dispose and are wired to the diodes in such a way that the oscillations that arise during the operation of the high-voltage split-diode transformer, in particular in the phase of blocking of the diodes, induces capacitive currents on the conductive coating, the currents are canceled mutually one to the other, in other words the sum of these currents is zero. This can be achieved, for example, in the fact that in the chambers the interference oscillations occur with identical amplitudes but in antiphase, and the capacitances between the chambers and the conductive coating are identical, with the result that the capacitive currents on the conductive coating compensate to the other. It is preferable to use an even number of cameras, which all have an identical number of turns, or at least an identical number of turns in pairs, with the result that the oscillations occur with quantized amplitudes. By virtue of the connections of the chambers to one another, and to the diodes, the oscillations with ascending and descending amplitudes occur in one direction, with the result that the compensation currents of two respective chambers whose oscillations have the same amplitudes are compensated to another, on the conductive coating. In this case, a group of cameras in the center of the high-voltage transformer has a pulse-free connection between two cameras that can advantageously be used for the focus connection of a kinescope. When the cameras are being coiled, it is necessary in this case to make sure that the cameras that are not yet full are not surrounded on both sides by wires, and that the sense of the winding of the cameras is uniform. Since the compensation currents cancel each other, the interference radiation of the oscillations arising in the high-voltage winding is effectively shielded, even if the ground connection for the conductive coating is omitted. The lower parts of the chambers have, in particular, the same thickness, for example 1 mm, with the result that the capacitances produced between the chambers and the conductive coating are identical. The final zero balance of the output currents can additionally be effected by different numbers of turns in individual chambers, whereby it is possible to reduce the remaining pulse voltages from, for example 40 V to about 0 V. for monitoring purposes, it is possible in this case to measure the compensation current between the conductive coating and a reference earth potential, for example earth. In the case of an ideal roll, the current decreases to zero. In the case of a high-voltage transformer that has two diodes, the cameras with the high-voltage winding are subdivided into three groups by the two diodes, the highest impulse voltages occurring on both sides through the two diodes and The focus connection are sent from the middle chamber, and they are free of pulse voltage. In the case of high voltage transformers having three and four diodes, a corresponding arrangement and wiring or winding of the chambers also makes it possible to achieve the result that the capacitive currents on the conductive coating compensate each other, with the result that that a ground connection can also be avoided in the case of these. In this case, the cameras are also preferably designed in such a way that the oscillations occur with the same amplitude but in antiphase. These also contain a middle group, with an even number of cameras, so that a focus voltage that is free of AC voltage can be sent. The high-voltage transformer present is thus excellently suited for recent televisions or monitor frames, since it operates practically without interference radiation. It is no longer necessary to worry that the interference radiation will interfere with the tuning circuits. The contact connection of the conductive coating, which is complicated with a reliable design and with this increases the cost of the high voltage transformer, can be avoided. The invention is explained below by way of example with reference to the diagrammatic drawings, in which: Figure 1 shows a block diagram with a split-diode high-voltage transformer having two diodes for generating a high voltage for a kinescope, Figure 2 shows a former for winding coils with windings and two diodes for a high voltage transformer, Figure 3 shows the circuitry of the cameras for a high voltage transformer having two diodes, Figure 4 shows a diagram of blocks with a high-voltage split-diode transformer that has three diodes to generate a high voltage for a kinescope; and Figure 5 shows the circuitry of the cameras for a high-voltage transformer that has three diodes. Figure 1 illustrates a split-diode high-voltage transformer Tr having a primary winding Wl and a high-voltage winding W2-W4 which is subdivided into partial windings 2, 3a, 3b and W4, a high-voltage diode 3 and 4 respective, for the purposes of rectification, which is interposed between the first and the second and the third and fourth partial windings. An intermediate connection F to provide a high voltage for the focus electrode of a kinescope 7 is placed between the second and the third high voltage windings W3a, W3b. One end of the partial winding W2 is connected to a reference potential G, usually the ground, and the high voltage UH that is sent in a connection for the operation of the kinescope 7 is present at one end of the partial winding W5. The high voltage is usually compensated by the capacitances of the cable of the connection cable and the capacitances in the kinescope 7, indicated here by the capacitance 6. This capacitance usually amounts to a number of nanofarads, so that the high voltage constitutes a potential of DC voltage for interference pulses of the high-voltage transformer. One end of the primary winding Wl is connected to a working voltage UB, and the other end is connected to a switching transistor 2, which is periodically turned on and off by a control signal 1. The high-voltage transformer additionally contains a core K , usually a ferrite core E / E or E / I. The transistor switch 2 is turned off in the short time of horizontal line reversal. This results in a high impulse load for the high voltage transformer Tr, and this load must be taken into account in the design of the transformer. Since the rectifier diodes 3, 4 are connected between the partial windings of the high voltage transformer in the assembly according to Figure 1, it is evident that the outer ends of the high voltage winding are free of AC voltage, since are connected to the potentials G and UH of DC voltage. Therefore, the impulse loads are mainly applied to the partial windings adjacent to the diodes, and are at a maximum, but antiphase, at the connections of diodes 3 and 4. The individual division of the impulse voltages is explained with reference to Figure 3. Figure 2 illustrates, in a sectional drawing, a winding former 9, which accommodates both the primary winding Wl and the high voltage winding subdivided into the partial windings W2-W4, which they are located below the primary winding Wl. The coil winding former 9 contains an axial internal cavity 11, which accommodates the ferrite core (not shown), and a multiplicity of cameras C, twelve in this example embodiment, the bottom of which approximately has a thickness of 1 mm in the direction of the cavity, and inside which the partial windings W2-W4 of the high-voltage winding are wound. In this case, three adjacent chambers respectively correspond to one of the partial windings W2, W3a, W3b and W4. An insulating layer 10, consisting of a number of layers of a laminar winding in this exemplary embodiment, is located above the chamber C. The primary winding Wl is wound on one or more layers wound directly directly on this insulating layer 10. In addition, auxiliary windings WH are applied to the primary winding Wl for the purpose of generating additional DC voltages. Examples of practical wire thicknesses are 0.335 mm or more for primary winding W1, and 0.05 mm enameled copper wire for high voltage winding. As an alternative to the laminar winding, a plastic sleeve is also possible as an insulating layer between the primary winding and the high-voltage winding, which can be pressed on the winding-up formers 9 with the high-voltage winding W2-W4. The primary winding can then be wound together with the auxiliary windings directly onto the plastic sleeve. By virtue of a corresponding arrangement of the diodes, described in PCT / EP 90/03882, the complete coil winding former can be kept very compact, even when a sleeve is used. The sleeve is then positioned in a positively immobilizing manner on the chambers C of the high-voltage winding W2-W4, and covers the latter completely. In this example embodiment, at the ends of the chamber, the coil winding former 9 has side edges 13 to accommodate the sheet winding 10 and the primary winding Wl. These elevated parts are followed, towards the outside, by two additional cameras 14, 16, which serve to accommodate the two high-voltage diodes 3, 4. The diodes 3, 4 are connected to the partial windings W2-W4 of the high-voltage winding via the wires of the corresponding windings. As a result of this design, the C-chambers with the high-voltage winding are completely covered by the primary winding Wl, separated by an insulating layer, with the result that the low-impedance primary winding Wl implements an effective shielding of the high frequency and intense interference that is produced by the switching of the switching transistor 2, and is stepped by the transformation ratio of the turns numbers of the primary winding Wl with respect to the high voltage winding. Silos diodes 3, 4 are in the off state, the interference oscillations are separated in different oscillations in each of the partial windings W2-W4, and the oscillation frequency in this case depends on the corresponding parasitic inductances, and the parasitic capacitances of each partial winding. In this example embodiment, the internal cavity 11 of the coil winding former 9, in which a limb of the core (not shown) is located, is provided with a conductive coating 15 on its entire surface, the conductive coating can be grounded , for example by contact with the nucleus. The conductive coating used can advantageously be a colloidal graphite layer, which can be applied in a spraying process, and has a high impedance conductivity. By this means, the interspace filled with inherently unavoidable air between the ferrite core and the winding conductor 9 is shielded against the high voltage, with the result that the formation of the crown is completely suppressed by this measurement. The conductivity of the coating is selected in such a way that eddy currents in the coating are avoided. The layer with the colloidal graphite can preferably be applied by means of a liquid spray containing colloidal graphite and adhesive in a solvent, and which additionally dissolves the plastic of the coil winding material 9 slightly, to increase the adhesion. This spraying can be applied in a simple manner, for example using a nozzle that sprays in the radial direction, and is conducted through the cavity 11 of the coil winding reel 9.

On its lower side, the coil winding device 9 contains electrical connections 12, by which the high voltage transformer is fixed directly on a circuit board. This will additionally be surrounded by a plastic housing (not shown) that is open towards the side of the connections, and is completely encapsulated together with the latter by means of a synthetic resin composition. The surface of the internal cavity 11 can, for example, also be provided with the conductive coating 15 by means of a metallized film, in particular plastic film. The metallized film is in this case wound in a superimposed manner between the core and the coil winding former, and should rest as tightly as possible with the metalized side on the surface of the internal cavity, so that corona effects are avoided . A low impedance metal sheet alone is not suitable, since it would form a short-circuit winding. A metallized plastic film, for example aluminized Mylar does not form a short-circuit winding on the periphery, even with superposition. Also conceivable is the use of two sheets, for example a plastic film and a sheet of metal, which are wound in a superimposed manner in such a way that the metal sheet has no electrical contact at the superimposition end. It is also possible to encapsulate the remaining cavity between the core K and the former for winding coils 9 with a material having a low conductivity. The structure and circuitry of the high-voltage winding of Figures 1 and 2 are explained in more detail with reference to Figure 3, which diagrammatically illustrates the windings in the Cl-C12 cameras, and also their circuit assemblies, without the coil winding former 9. The first partial winding W2 contains the three cameras Cl-C3, which are connected in series, where the start of the camera Cl is connected to ground G, and the end of the camera C3 is connected to the diode 3. The partial windings W3a and W3b are located in cameras C4 -C6 and C7-C9, respectively, and are also connected in series. The partial winding W4 contains the CIO-C12 cameras, the connection for the high voltage UH is sent from the end of the C12 camera. The start of the camera C4 is connected to the cathode of the diode 4, and the end of the camera C9 is connected to the anode of the diode 3. The anode of the diode 4 is connected to the start of the CIO camera. In this example embodiment, all the cameras contain approximately the same number of turns, which amounts to approximately 300, by way of example, given a high voltage to be generated of 24 kV. As a result of this symmetric structure, the following conditions are produced for the pulse voltages UP: since the diodes 3, 4 are symmetrically connected with respect to the ground G and the high voltage UH, and also with respect to the center of the winding of high voltage, the identical impulse voltages, which are approximately +/- 6 kVpp given a high voltage of 24 kV, are present through the two diodes. These voltages are present correspondingly in cameras C3, C4, C9 and CIO. Since the cameras are connected in series, the voltage for the remaining chambers is correspondingly reduced according to the voltage dividing principle, in which case, in this example embodiment, an impulse voltage of 2 kVpp is present per camera of agreement. with the winding between the bottom of the camera and the top of the camera. The impulse voltages UP +2, * 4 and +6 kV are therefore present in the lower part of the camera of the cameras C1-C3, since the diode 3 is connected to the lower part of the camera of the camera C3. In this case, these cameras are wound in the order C3, C2, Cl, with the result that the winding end of the Cl camera, the upper part of the chamber, is connected to ground G. The impulse voltages 0, -2 and -4 kV are present in the lower parts of the cameras of cameras C12, Cll, CIO, since these are coiled starting with the camera C12, and the end of the wire of the camera C12 is sent to the high voltage connection UH, and to the end of the wire of the CIO camera for the connection to the diode 4. In the case of the cameras C4-C9 , the corresponding impulse voltages of +4 - -6 kV with a voltage difference of 2 kV per chamber are established in the lower parts of the chambers, since the bottom of the chamber of the camera C9 is connected to the cathode of the diode 3, and the winding end of the camera C4 is connected to the anode of the diode 4. The connection between the cameras C6 and C7 is free of impulse voltage, and is therefore used for the focus voltage F. The winding of high voltage is subdivided by diodes 3, 4 as it was in groups C1-C3, C4-C9 and C10-C12, in each group the pulse voltages UP assume quantized values in an ascending or descending sequence, and an amplitude value of zero, which can be used for the focus connection, which occurs in the or a middle group C4-C9. The pulse voltages UP in the lower part of the chamber of the cameras C1-C12 therefore produce the sum of zero. Since the thickness of the lower parts of the chambers towards the conductive coating 15 is selected to be identical for all the chambers in this example embodiment, the capacitances SC between the windings C1-C12 of the chamber and the conductive coating 15 are also all identical, ignoring the marginal effects. The capacitive currents induced by the pulse voltages UP on the conductive coating 15 are therefore proportional to the quantized pulse voltages UP and therefore also produce the sum of zero. As a result of this, the cameras C1-C12 are shielded by the conductive coating 15 just as effectively as if the latter were provided with a ground connection G. The latter can therefore be discarded. The circuit of Figure 4 illustrates a split diode transformer having three diodes 3-5 that is constructed in a manner similar to the high voltage transformer explained with reference to Figures 1 and 2. In the Figures, therefore, it is they provide identical concepts with the same reference symbols. A respective diode 3, 4, 5 is arranged between the partial windings W2-W5, and the intermediate connection F for the focus electrode is in this case sent from the partial winding W3, as explained below with reference to Figure 5. Figure 5 shows a high-voltage winding having 12 cameras C1-C12, according to the example embodiment illustrated in Figure 4, which is subdivided by diodes D3-D5 into four partial windings or groups of cameras C1-C2, C3-C6, C7-C9, C10-C12. By virtue of a corresponding arrangement and dimensioning of the cameras C1-C12 with respect to the diodes 3-5, here also values of quantized amplitude A, from -2 to +2, are produced and by virtue of a corresponding dimensioning of the parameters of the former for winding coils, the capacitances between the lower part of the chamber and the conductive coating 15 are in each case identical for each chamber C1-C12, so that the A values of quantized amplitude, as specified in Figure 5, they produce the sum of zero, and the capacitive currents on the conductive coating 15 likewise cancel each other. As a result of this, the ground connection G can also be omitted in this case. In this case, the cameras are coiled starting with the camera Cl in ascending order until the camera C12, all the connection wires for the diodes 3-5 are sent downwards, in the Figure, so that the three diodes 3- 5 in this case they are located below the Cl camera. For high voltage transformers that have more than three diodes, the coil winding formers and the high voltage windings can also be constructed in such a way that the sum of the capacitive currents on the conductive coating results in zero, so that these, too, are shielded by the conductive coating, and are free of radiation. Due to relatively small asymmetries, for example marginal effects, specific cameras can, under certain circumstances, not produce exactly the desired amplitude values of the impulse voltages, thus necessitating fine adjustments. This can be effected, for example, by these cameras that have number of turns that are changed accordingly. This means that for these cases, too, the capacitive currents on the conductive coating can be reduced to practically zero.

The structure used in the example embodiment mentioned above, with an identical thickness of the lower parts of the chambers and an approximately identical number of turns for all the chambers C1-C12, is not a necessary precondition for the capacitive currents induced on the cladding driver 15 cancel each other. By way of example, it is also conceivable for two cameras in each case that are constructed in an identical manner, and arranged symmetrically with respect to the diodes, in such a way that the capacitive currents on the conductive coating 15 in each case cancel each other out. for these, for example to provide better high voltage intensity for specific cameras. Further exemplary embodiments are also possible, the chambers have to be constructed and arranged in such a way that the sum of all the capacitive currents on the conductive coating 15 results in zero, or the capacitive currents mutually compensate each other. The embodiments described above of a split-diode high-voltage transformer are only by way of example; in particular, the high-voltage winding can also be subdivided into more than four partial windings if more than three diodes are used, and also in a different number of cameras C. The circuits of the class illustrated in Figures 1 and 4 are also used in computer monitors.

Claims (11)

1. CLAIMS 1. High voltage split-diode transformer that has a core, a primary winding and a high-voltage winding, which is arranged in a coil winding chamber, in which the cameras with the high-voltage winding are located below the primary coil, a conductive coating is disposed on the surface of the internal cavity of the coil winding former, and by virtue of a corresponding arrangement and wiring of the chambers, the oscillations that arise during operation in the high voltage transformer. voltage induces capacitive currents on the conductive coating, the sum of the capacitive currents results approximately zero.
2. 2. High voltage transformer according to claim 1, characterized in that by virtue of a symmetrical arrangement and cabling of the cameras with respect to the diodes, the oscillations induce capacitive currents on the conductive coating that occur in pairs, with the same amplitude but in antiphase, and through this they cancel each other.
3. 3 High voltage transformer according to claim 2, characterized in that the number of cameras is even, and two cameras in each case are full and connected to other cameras, in such a way that the interference pulses produced in these cameras each have an identical amplitude but they are in antiphase.
4. 4. High voltage transformer according to claim 3, characterized in that, in the case of at least two chambers, the lower parts of the chambers have approximately the same thickness, and their cabling has an identical number of turns, with the result that The capacitances between these chambers and the conductive coating and also the capacitive currents induced on the conductive coating are in each case approximately identical in terms of their magnitude.
5. 5. High voltage transformer according to claim 4, characterized in that the number of high voltage transformer chambers is even.
6. 6. High voltage transformer according to claim 4, characterized in that the high-voltage winding is subdivided into groups of cameras, two groups in each case are connected to each other by a diode, with the result that the end of the first group, in the direction of the winding, it is connected to the end of the second group via a diode, and the start of the second group is connected via a diode to the start of the third group, and because a middle group has an even number of cameras, a focus connection it is sent from the center of this group.
7. 7. High voltage transformer according to claim 6, characterized in that the high voltage transformer has two diodes that subdivide the high voltage winding into three groups, the first and third groups have an identical number of cameras, and the middle group has a pair number of cameras and the focus connection.
8. 8. High voltage transformer according to claim 4, characterized in that the number of diodes is three, which subdivide the high-voltage winding into four groups, the number of cameras in the four groups is two, four and two times three, and the The second group has a focus connection that is sent from the center of this group.
9. 9. High voltage transformer according to any of the preceding claims, characterized in that the high voltage diodes are arranged laterally with respect to the chambers, and because the primary winding completely covers the high voltage winding.
10. 10. High voltage transformer according to any of the preceding claims, characterized in that the first and last high voltage winding chambers are at the ground potential in terms of the DC voltage.
11. 11. High voltage transformer according to any of the preceding claims, characterized in that the final zero balance of the capacitive currents is produced by changed number of turns in individual chambers. SUMMARY OF THE INVENTION The present invention specifies a high-voltage split-diode transformer having a core, a primary winding and a high-voltage winding, in which it is arranged in chambers (Cl-C12) of a coil winding former, in which the cameras (C1-C12) with the high-voltage winding are located below the primary winding, a conductive coating (15) is disposed on the surface of the internal cavity of the coil-winding former, and by virtue of a corresponding arrangement and wiring of the cameras (C1-C12), the oscillations that arise during the operation in the high-voltage transformer induce capacitive currents on the conductive coating (15), the sum of the capacitive currents is approximately zero. This can be achieved, for example by virtue of a symmetrical arrangement and wiring of the cameras (C1-C12) with respect to the diodes (3,4), the oscillations induce capacitive currents on the conductive coating (15) that occur in pairs with the same amplitude, but in antiphase, and through this they cancel each other. In particular, by virtue of an identical thickness of the lower part and approximately identical number of turns for all the cameras (Cl-C12), the capacitive currents occur with a quantized amplitude, with the result that the values thereof can be defined in a simple way, since the parasitic capacitances (SC) are identical for all cameras (C1-C12). This arrangement allows the ground connection (G) to be omitted, although the shielding effect of the conductive coating (15) is retained.